Critical dimension measuring instruments are used in the production of semiconductors to measure critical dimensions (cd), in order to check the dimensional consistency of features on semiconductor chips after the individual production steps, and to control the quality of the production steps. As packing densities on semiconductor chips steadily increase, the critical dimensions of the features to be measured are simultaneously becoming smaller and smaller. Requirements in terms of the specifications of measurement and inspection systems, e.g. the measurement accuracy and repeatability of measurement results, are becoming correspondingly more stringent.
Optical scanning methods and corresponding optical measurement apparatuses are preferred in production, even though the critical dimensions of the features to be measured are already smaller than the optical wavelengths used for measurement or inspection. The reason for this is that optical measurement apparatuses are substantially easier to use than non-optical ones. At the same time, however, the demands on optical measurement apparatuses are increasing, especially with regard to resolving power and the separation of adjacent features. The illumination quality of the features being examined plays a critical role here. To allow critical dimensions, edge profiles, and similar critical variables to be measured reproducibly using optical means, Fourier optics demands that the illumination of measurement fields, i.e. of the features acquired during measurement and their surroundings, be extremely homogeneous.
In known critical dimension measuring instruments, this is achieved using various kinds of illumination devices. In one type of illumination device, for example, gas discharge lamps are used as light sources, and the specimens to be examined are illuminated using classic Köhler illumination. The known gas discharge lamps exhibit near and far light intensity distributions that are unfavorable for critical dimension measurement (i.e. asymmetrical). These field distributions result, in the context of Köhler illumination, in an inhomogeneous illumination of the optical measurement field and the pupil. The “pupil” is defined in an incident-light illumination system as the objective pupil, and in a transmitted-light illumination system as the condenser pupil. As a result of the inhomogeneous and, in particular, oblique illumination, the critical dimensions that are measured depend on the position and orientation, within the optical measurement field, of the feature being measured. This has the disadvantage that the user's stringent requirements, in terms of reproducibility and measurement results that are independent of the feature's measurement position and orientation, cannot be met.
In other critical dimension measuring instruments, lasers are used as light sources. This has the disadvantage that the radiation emitted by the laser is almost completely coherent. This high level of coherency results in internal interference in the radiation field. This is perceptible, for example, as laser light granulation in the measurement field, and likewise has a negative effect on critical dimension measurements, making it impossible to achieve high reproducibility and measurement results that are highly independent of the feature's measurement position and orientation.